1. Field of the Invention
The present invention relates generally to the drilling of oil and gas wells, and more particularly, but not by way of limitation, to a method of preventing drilling fluid overflow and the occurrence of uncontrolled formation fluid blowouts when removing drill pipe sections from a drill string disposed in a well bore.
2. Description of the Prior Art
In the drilling of well bores penetrating subterranean formations, uncontrolled formation fluid blowouts can occur when a formation is penetrated containing fluids at pressures exceeding the hydrostatic head exerted on the formation by the column of drilling fluid contained in the well bore. While mechanical blowout preventer apparatus are commonly used in well bore drilling operations which function to shut off the annulus between the walls of the well bore (the surface casing) and the drill string disposed therein, such apparatus are effective in stopping blowouts only after they have commenced, often resulting in expensive and time-consuming delays while heavy fluid is pumped into the well bore to increase the hydrostatic head on the penetrated formation and kill the well.
One of the main causes of formation fluid blowouts during drilling operations is the failure to adequately compensate for the loss of drilling fluid level in the well bore during the withdrawal of the drill string therefrom. It is frequently necessary to withdraw the drill string from the well bore to change the drill bit, replace worn out drill pipe sections, etc. Because during drilling, drilling fluid is circulated down the drill string, through ports in the drill bit and up the annulus between the drill string and the walls of the well bore to the surface, when removal of the drill string is necessary the drill string and the annulus are completely filled with drilling fluid. Generally for economic reasons the density of the drilling fluid utilized is maintained at a minimum level but such that the hydrostatic pressure exerted by the column of drilling fluid on the bottom of the well bore exceeds formation fluid pressures which are encountered thereby preventing formation fluids from entering the well bore. During drilling when the drill string and annulus are filled with drilling fluid the required hydrostatic head is relatively simple to maintain. However, during the removal of the drill string from the well bore when the circulation of drilling fluid is stopped, the level of drilling fluid within the well bore can very easily be inadvertently allowed to lower to a point whereby the hydrostatic head exerted at the bottom of the well bore is lower than the pressure of fluids contained in a formation penetrated by the well bore resulting in the formation fluids entering the well bore and flowing up the annulus, i.e., a blowout which can result in the loss of life, energy, material and cause damage to the environment.
Another problem that often occurs during the withdrawal of the drill string from the well bore is the overflow of drilling fluid through the drill string as drill pipe sections are disconnected therefrom resulting in the loss of drilling fluid and the lowering of the level of drilling fluid in the well bore. Such overflow is due to the drilling fluid in the annulus being of a higher density than the drilling fluid within the drill string which causes the level of drilling fluid in the drill string to be higher than the level in the annulus. That is, because the drilling fluid is circulated while drilling, the fluid inside the drill string is relatively free of drilled solids while the fluid in the annulus is carrying newly drilled solids to the surface for removal. In addition, the drill bit at the bottom of the drill string includes nozzles of restricted cross-sectional area which inhibit the drainage of the drill string and the equilization of the levels of drilling fluid in the drill string and the annulus. When the circulation of drilling fluid is stopped for the removal of the drill string, the combination of the above factors results in the level of drilling fluid in the drill string being above the point at which drill pipe sections are disconnected from the drill string which in turn causes drilling fluid to overflow from the drill string at the point of disconnection. As is well understood by those skilled in the art, in removing drill pipe sections from the drill string, the drill string is raised within the well bore which lowers the level of drilling fluid therein. Thus, during the withdrawal of the drill string, overflow initially occurs which lowers the level of the drilling fluid in the well bore, and because drilling personnel are reluctant to refill the annulus with drilling fluid which will cause additional overflow (even though they are generally directed to do so) the continued withdrawal of the drill string from the well bore results in a further lowering of the level of drilling fluid therein, often to the extent of allowing formation fluids to enter the well bore and a dangerous blowout condition to occur.
by the present invention a method of preventing drilling fluid overflow and the occurrence of uncontrolled formation fluid blowouts during the removal of the drill string from a well bore is provided. The practice of the method brings about the accurate and constant control of the level of drilling fluid within the well bore during withdrawal of the drill string thereby insuring the prevention of uncontrolled blowouts and eliminating undesirable overflow.